System for providing flow-targeted ventilation synchronized to a patients breathing cycle

ABSTRACT

An open system provides breath-synchronized, flow-targeted ventilation to augment respiration by a self-breathing patient. A sensor detects a physical property of a patient&#39;s respiratory cycle. A processor monitors the sensor and controls a gas source to deliver oxygen-containing gas through a tube extending into the patient&#39;s airway with the flow rate varying over each respiratory cycle in a predetermined non-constant waveform synchronized with the respiratory cycle to augment the patient&#39;s spontaneous respiration. Gas is delivered at a flow rate sufficient to significantly mitigate the airway pressure the patient must generate during spontaneous breathing and thereby reduce the patient&#39;s work of breathing.

RELATED APPLICATION

The present application is a continuation of the Applicants' Ser. No.13/189,956, entitled “System For Providing Flow-Targeted VentilationSynchronized To A Patient's Breathing Cycle,” filed on Jul. 25, 2011,now U.S. Pat. No. 8,651,105, issued on Feb. 18, 2014, which is acontinuation of U.S. patent application Ser. No. 11/627,512, filed onJan. 26, 2007, now U.S. Pat. No. 8,020,558.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of mechanicalventilation of patients. More specifically, the present inventiondiscloses an open system for providing ventilation in a predeterminedflow waveform synchronized to a patient's breathing cycle to augmentrespiration by a self-breathing patient.

2. Statement of the Problem

Standard mechanical ventilators deliver pressure. There are threeclassifications of mechanical ventilators that are based upon how theyadminister pressure ventilation. Negative pressure ventilation requiresan apparatus that expands the chest wall, creating levels ofsub-atmospheric pressure that draw air or oxygen-enriched ambient gasthrough the upper airway and into the lungs. Positive pressureventilation requires that supra-atmospheric pressure is generated andcontrolled by the device so that air or oxygen-enriched air ispressurized to the degree that it can be forcibly driven through theupper airway and into the lungs. The third method is a combination ofpositive/negative pressure. The prime example is a high frequencyoscillator, where oscillations of negative and positive pressure areproduced in the airway in a sinusoidal pattern that is independent ofself-breathing efforts and at a rate that exceeds the maximum humanrespiratory rate by many fold.

Positive pressure ventilators are by far the most frequently usedmechanical breathing device. They can be further divided into invasiveor noninvasive systems. Invasive systems utilize an endotracheal ortracheostomy tube, with an inflated tracheal cuff that creates anobstruction closing off the upper airway from atmospheric or ambient gasand thus creates a closed system between the positive pressureventilator and the lungs. This is Closed System Positive PressureVentilation (CSPPV). Breath delivery with positive pressure ventilatorscan be categorized as either pressure-targeted or volume-targetedventilation. Generation of a specific airway pressure on inspiration andoften a different pressure on expiration are pressure-targeted outcomes,or alternatively, a level of pressure is generated to achieve theprimary goal of a targeted tidal volume delivered to the lungs(volume-targeted ventilation). The closed system allows a positivepressure breath to be delivered through the inspiratory valve of thedevice, through the inspiratory limb of the breathing circuit anddirectly to the lungs without loss of pressure by dissipation of gasinto the atmosphere. The delivery of the breath can be forced into thepatient independent of the patient's breathing pattern (time triggering)or synchronized with the patient's effort to inhale (pressure or flowtriggering), but the patient's normal negative pressure inspirationduring self-breathing is lost as it is converted to a positive pressurebreath. Peak inspiratory airway pressures of 20 to 30 cm H₂O or greaterare commonly achieved. The inspiratory valve is open during thepatient's entire inspiratory phase. During inspiration the expiratoryvalve on the expiratory limb of the breathing circuit must remain closedto maintain the pressurized breath. The transition from inspiration toexpiration is ultimately governed by the ventilator (breath cycling) andnot the patient, because in a closed system, the expiratory valve mustopen to allow exhalation. During exhalation, the inspiratory valve isclosed to prevent retrograde flow of gas back into the machine, whichcould result in the physiologic terms of rebreathing carbon dioxide ordead space gas, which is dangerous and potentially life-threatening. Theexpiratory valve is at least partially open to allow the breath toadequately vent into the atmosphere. The pressure at the onset ofexhalation with CSPPV approximates the peak inspiratory pressure (e.g.,20 to 30 cm H₂O or greater) and dissipates over the expiratory phase asa function of the patient's exhaled gas being allowed to exit throughthe exhalation valve. Though the expiratory valve or mechanism is oftencompletely open during exhalation, partial closure of the expiratoryvalve or mechanisms during one or more components of the expiratoryphase may achieve a targeted level of expiratory pressure within thelungs while still maintaining adequate exhalation through the valve andacceptable gas exchange. Examples of methods to achieve pressure in thelungs during exhalation due to partial exhalation valve closure duringone or more points in exhalation include expiratory retard and positiveend expiratory pressure (PEEP). During CSPPV, the expiratory valve isnot completely closed during exhalation as life-threatening excessivepressure and suffocation could result. Partial closure of the expiratoryvalve during end exhalation (PEEP) prevents the decline in airwaypressure from ever returning to the 0 cm H₂O baseline between exhalationand inhalation. Prescribed PEEP may be 5 to 15 cm H₂O or more. Onexpiration, with CSPPV all the exhaled gas is routed through theexpiratory limb of the circuit and is available to the ventilator foranalysis. This analysis is required for proper ventilator function andmonitoring. More than one CSPPV mode can be administered simultaneously(e.g., intermittent mandatory ventilation with pressure support andpositive-end expiratory pressure).

Though CSPPV can be life-saving for patients who are unable to do anynegative pressure self-breathing, there are a number of problems withthe CSPPV technology. It has been scientifically demonstrated that thepressure generated by positive pressure ventilation can injure thedelicate structures of the lungs. This injury can cause significantmorbidity and mortality, particularly when CSPPV is superimposed uponacute lung injury from pneumonia or adult respiratory distress syndrome(ARDS). Over time, positive pressures that were once thought to be safehave been determined to cause lung injury. The safe positive pressurethreshold that does not cause (or worsen) acute lung injury on somelevel is presently unknown. There is a scientific trend fordocumentation of acute lung injury with lower and lower positivepressures as more is learned about the pathophysiology of acute lunginjury on organ, tissue, cellular, biochemical and genetic levels. Incertain clinical settings positive inspiratory and/or expiratorypressures may impair gas exchange in the lungs. Positive pressureventilation can cause life-threatening impairment of cardiac output andcan cause lung collapse (tension pneumothorax) resulting frombarotrauma.

Other issues associated with the endotracheal tube with inflated cuffthat is required to maintain pressure with CSPPV can also cause seriousinjury such as tracheomalasia, tracheoesophageal fistula, trachealgranulation tissue and tracheal stenosis and necrosis. Along withserious injury, the inflated tracheal tube cuff has other consequences.Its use does not allow for communication between the larynx and theupper airway and causes gas to be channeled away from a patient'snatural humidification system. This is detrimental to the patient. Theinflated cuff prohibits utilization of the vocal cords. Patients areunable to speak causing poor communication between the patient andhealthcare providers and family thus impeding proper informed consentand establishment of advanced directives. This absence of speech cancause frustration, anxiety and depression. Bypassing the larynx alsoimpairs cough. Normal closure of the vocal cords allows generation ofthe glottic blast that facilitates effective cough and clearance ofrespiratory secretions. Finally, the vocal cords serve as a variableregulator of respiratory flow that fine tunes passage of gas in and outof the lungs to optimize gas exchange.

Use of an inflated cuff also pools upper respiratory secretions abovethe cuff and, over time, these secretions become contaminated withbacteria. These pooled secretions can leak into the lungs and increasethe risk of ventilator-associated pneumonia (VAP). Additionally,inflated tracheal tube cuffs can impair swallow. Though tracheostomytubes may be more comfortable, endotracheal tubes that pass through thenose or mouth and into the trachea are typically used initially and arevery uncomfortable. Tracheostomy tubes require a surgical procedure thatcan result in a number of complications, including bleeding, infection,barotrauma and airway obstruction.

Liberating patients from CSPPV requires a successful return of thepatient to normal negative pressure self-breathing. This has proven tobe difficult, particularly when patients have had their breathingcontrolled and altered by CSPPV for greater than 21 days (prolongedmechanical ventilation, or PMV). In fact, once patients have requiredCSPPV for greater than 21 days, the wean success rate is only about 50%overall, with a range of 35% to about 60%.

In the past, efforts have been made to allow patients to speak throughthe upper airway during CSPPV. One method utilizes deflation of thetracheal tube cuff. This open system was not intended for use withstandard CSPPV as inadequate ventilation can result. Use of this methodmay allow a significant portion of the ventilator breath intended fordelivery entirely to the patient's lungs to escape through the upperairway. Modes that function based upon targeting a specific inspiredtidal volume or pressure and expired pressure can be significantlycompromised by the absence of connectivity of the ventilator to closedpassages in and out of the lungs and inability of pressures measured inthose passages to accurately reflect lung pressure, since pressure canfreely dissipate via the upper airway. The volume and characteristics ofthe exhaled breath passing back to the ventilator in this type of opensystem will be inaccurate and could compromise the ventilator's abilityto use properties such as exhaled tidal volume or airway pressure toevaluate proper ventilator delivery. Unnecessary triggering of alarmssuch as low pressure or inadequate expired breath volume may occur.Absence of a closed system may result in dys-synchrony between theventilator and the patient if adequate feedback is not received by theventilator regarding the patient's breathing efforts thus the ventilatoris unable to adequately assess when to deliver the breath or when toterminate the breath. Such ventilator-patient dys-synchrony is known tohave deleterious physiologic effects. Some modes such as pressuresupport ventilation may partially or completely compensate for the leakwith the deflated cuff by increasing device driving pressure to maintainthe desired pressure delivered to the lungs. However, use of thepositive pressure ventilator by this method is still positive pressureventilation and still pressure-targeted and not flow-targeted. Thepatient's negative pressure self-breathing is still converted topositive pressure breaths.

When the cuff is deflated, one-way inspiratory valves may be placed inthe external CSPPV circuit to prevent gas from flowing back through themachine on exhalation. This is intended to direct expired gas up throughthe vocal cords to facilitate speech. However, many of the problemsnoted above that are encountered with using standard closed system CSPPVwith an open system still remain. Use of the positive pressureventilator by this method is still positive pressure ventilation andstill pressure-targeted or volume-targeted and not flow-targeted. Thepatient's negative pressure self-breathing is still converted topositive pressure breaths.

Noninvasive ventilation does not require a tracheal tube with inflatedcuff. Bi-level positive pressure ventilation generates a set positivepressure on both inspiration and expiration as the targeted outcomes(pressure-targeted ventilation). An interface such as nasal ornasal/oral mask, full face mask or nasal pillows or similar devices arerequired to create a barrier between the upper airway and theatmosphere. As with CSPPV, the patient's negative pressureself-breathing is converted to positive pressure breaths. Theobstruction created by the interface is great enough that it allowsgeneration of adequate positive pressure to provide all or nearly all ofthe patient's required breath. The obstruction created by the interfaceallows lung-distending pressure to be maintained on exhalation which isalways less than the pressure on inhalation. The patient both inhalesand exhales through the single-limb breathing circuit. As with CSPPV,exhalation of gas back into a circuit maintains proper ventilatorfunction and monitoring. The gas delivery valve connected to the singlelimb breathing circuit is continuously open during inspiration andexpiration to maintain desired inspiratory and expiratory positivepressures. The pressurized breath is triggered either when the patientmakes an effort to breathe or time-triggered when a breath is not sensedand a specified time has passed. This prior art is referred to asSubstantially Closed System Positive Pressure Ventilation (SCSPPV).Since the interface is usually not completely sealed, there is a leak ofpressure and loss of a portion of the gas into the atmosphere. Inaddition to gas leak through the upper airway, the single limb circuithas an exhalation valve at the distal end that is constantly open duringboth inhalation and exhalation. The valve remains completely open duringinspiration to flush CO₂ from the circuit and completely open onexpiration to allow the exhaled breath to escape into the atmosphere.Rebreathed CO₂ can cause significant morbidity and mortality. Commonexhalation valves or mechanisms include Whisper Swivel, Castle Port orNRV devices. Under no circumstances should the valve be occluded. Themicroprocessor constantly evaluates leaks by comparing pressure in thelumen of the proximal inspiratory circuit to pressure as measuredthrough monitoring tubing in communication with the lumen of the distalend of the circuit. The microprocessor can also compensate by changingthe driving pressure to maintain the primary target of deliveredpressure (pressure-targeted ventilation) during inspiration andexpiration.

With SCSPPV, the absence of a tracheal tube with an inflated cuff avoidsa number of the previously described complications of CSPPV. However,the nasal or nasal/oral mask, full face mask or nasal pillows or otherdevices used with SCSPPV to serve as an interface between the upperairway and atmosphere can be uncomfortable. Skin ulcerations andabrasions can result from tight-fitting masks. Patients in respiratorydistress may feel claustrophobic with these devices covering their nose,mouth or entire face. The devices can make speech difficult and eatingand swallowing difficult as well. Devices covering the mouth or face canimpair the patient's ability to expectorate sputum. Similar to CSPPV, adisadvantage of SCSPPV technology is the requirement for generation ofpressure. Depending on the device, peak inspiratory pressures of up to20 to 40 cm H₂O can be generated. The pressurized breath delivery can beuncomfortable. Current medical literature shows that SCSPPV may beneither effective nor tolerated in the extremes of mild and severerespiratory failure. In certain clinical settings positive inspiratoryand/or expiratory pressures may impair cardiac output and gas exchangein the lungs. Though not as likely to occur as with CSPPV, acute lunginjury and barotrauma may potentially occur when pressures in the highrange are delivered. Similar to CSPPV, the use of SCSPPV eliminatesnormal negative pressure self-breathing. As with CSPPV, successfulreturn to normal negative pressure self-breathing is required forsuccessful discontinuation of SCSPPV.

Transtracheal augmented ventilation (TTAV) is a prior art that is analternative to positive pressure ventilation. TTAV is not intended togive full ventilatory support like a CSPPV device, but augments thepatient's self-breathing by utilizing an open system and delivering aconstant and continuous flow of about 8 to 20 L/min of a heated andhumidified air and oxygen blend to the lungs during both inspiration andexpiration. It is an open system because there is no inflated trachealcuff and no mask, nasal pillows or other device to create a complete ornear complete barrier between the mouth and/or nose and the atmosphere.Because of the nature of the open system, delivered gas can easilyescape into the atmosphere and positive pressure is not a targetedoutcome. Tidal volume that the patient inspires through the device isnot an outcome that can be reliably targeted because of volume lossthrough the upper airway and variability of volume that the patientinspires through the upper airway during negative pressureself-breathing. In fact, TTAV is only intended for use on patients whoare able to do some degree of negative pressure self-breathing. Benefitsfrom augmented ventilation are derived from a defined constant andcontinuous flow that is superimposed upon the patient's own breathingcycle. Patients can freely inhale room air through the mouth and nose inaddition to the gas delivered by the TTAV device. With prior art, air oroxygen enriched air can be delivered directly into the trachea via atranstracheal catheter. The delivery device heats and humidifies the gasto eliminate complications and sequellae from the humidity deficit thatwould otherwise occur from delivering constant and continuous flows of 8to 20 L/min of dry cool gas directly into the trachea. There is a singleinspiratory circuit with no expiratory circuit or expiratory valvebecause the patient is free to exhale normally through the nose andmouth. No inspiratory valve is used as a constant and continuous flow isdelivered to the patient rather than distinct breaths. Since theconstant and continuous flow is superimposed upon the patient's inherentnegative pressure self-breathing cycle, synchronization with thepatient's breathing is not required. A pressure relief valve preventsover-pressurization within the device in the event of a malfunction orobstruction and an alarm signals the event. Exhalation of gas back intothe breathing circuit or into the device is not required to monitor ormanage gas delivery during routine operation.

Compared to either low flows used with prior art transtracheal oxygentherapy or mouth breathing without transtracheal flows, potentialphysiologic benefits of TTAV at a constant continuous flow of 10 L/mininclude correction of hypoxemia, reduced inspiratory work of breathing,decreased volume of gas the patient must inspire through the upperairway, and improved exercise capacity. The effect of constantcontinuous TTAV flow above 10 L/min corrects hypoxemia. Since priorstudies show that the relationship between flow and response is directlyrelated, one would predict improved response in terms of reducedinspiratory work of breathing, decreased volume of gas the patient mustinspire through the upper airway, and improved exercise capacity withflows above 10 L/min. However, the effect on these specific physiologicparameters has not been specifically evaluated. Compared to low flowtranstracheal oxygen therapy at 1.5 L/min, potential physiologicbenefits of TTAV at a constant and continuous flow of 15 L/minadditionally include increased efficiency of breathing, reduced totalminute ventilation and reduced end-expiratory lung volume. The effect ofconstant and continuous TTAV flow above 15 L/min on these physiologicparameters has not been evaluated. Reduced physiologic dead space isseen with low flow transtracheal oxygen (up to 6-8 L/min) as compared tomouth breathing. However, it is not known if constant and continuousflow above 8 L/min with TTAV results in any further reduction inphysiologic dead space. TTAV at 10 L/min as a means of augmentingventilation of patients with chronic respiratory failure duringnocturnal home use has been shown to be safe and effective. Furthermore,removal of prolonged mechanical ventilation patients from CSPPV andplacement on a constant and continuous TTAV flow from 10 to 15 L/minthrough a catheter placed within the lumen of a deflated cufftracheostomy tube has been shown to improve wean success from CSPPV. Inthis setting, use of the TTAV device and CSPPV device are alternated inan iterative fashion, with a progressive increase of time on TTAV. Withthe cuff deflated while the patient is on the TTAV device with aconstant and continuous flow of 10 L/min, all gas is expired through theglottis and upper airway resulting in the previously described benefitsassociated with restored speech, more effective cough and return ofglottic function as a physiologic variable regulator of respiratoryflow. As noted previously, the inflated tracheal cuff prevents thesebenefits from occurring with CSPPV. It is unknown if constant andcontinuous TTAV flow above 15 L/min improves effectiveness or weanoutcome.

A less than optimal condition associated with TTAV is that a constantand continuous flow is administered throughout the inspiratory andexpiratory phases of the respiratory cycle. Each of the potentialbenefits as described above will likely have different respiratory cycletargeted flow rates and waveforms to achieve maximal beneficial effectin a given patient, and requirements may change with alterations in theclinical status of that individual over time. Additionally, patientswith different diseases or disorders may benefit more from certainphysiologic effects than from others, and those effects can beinfluenced by different flows and flow waveforms administered inspecific phases (or phase components) of the respiratory cycle.Synchronizing the amount and pattern of flow with specific phases of thebreathing cycle or even components of phases of the breathing cycle maymarkedly influence clinical efficacy. In contrast, constant continuousflows delivered throughout the inspiratory and expiratory phases as seenin the prior art may not be efficacious. For example, a constant andcontinuous flow of 40 L/min delivered throughout the inspiratory phaseof breathing may significantly increase total inspiratory work ofbreathing rather than reduce it if the specific physiologic effect onthe respiratory inspiratory phase and phase transitions as well as thephase components are not considered. With prior TTAV art, that constantand continuous flow of 40 L/min would also be delivered duringexhalation. That amount of flow throughout expiration would likelyimpose a significant expiratory workload causing the patient to forciblyexhale against the constant incoming stream of tracheal gas. This couldresult in respiratory muscle fatigue and impaired gas exchange. Theremay be benefit to transiently interrupting flow during certaincomponents of the breathing cycle which could influence clinicalefficacy. TTAV with a constant and continuous flow eliminates thepotential for improving safety, efficacy and tolerance by the inabilityof the prior art to target non-constant, potentially non continuousflows with different peak flows and flow patterns that are strategicallysynchronized with the various phases or components of the phases of apatient's breathing cycle.

Another weakness associated with conventional TTAV systems is that,other than an alarm and pressure relief valve for excessive pressuresencountered within the channels of the delivery device and lumen of thecircuit, there are no sensors or measurement devices that providephysiologic data that identify phases or components of phases of thepatient's negative pressure self-breathing cycle that are designed toregulate breath synchronized, flow-targeted delivery. Conventional TTAVsystems do not have microprocessors supporting breath-synchronized,flow-targeted delivery designed to manage patient physiologic data,display the data, trigger alarms for out of range results or incorporatethat information into intelligent processing for a feedback loop orservo controlled device response to the physiologic data. Anotherproblem with conventional TTAV systems is that the only clinicalimplementation to date has been limited to use with a transtrachealcatheter.

The prior art also includes ventilation systems based on “flowtriggering” a breath that is subsequently supported by CSPPV. As opposedto a drop in circuit/ventilator pressure indirectly indicating a breatheffort by a patient, the CSPPV breath is triggered by a presumed effortby the patient to generate inspiratory flow. Though patient inspiratoryflow is not directly measured, the breathing effort is presumed becauseflow inside the expiratory limb is measured to drop to less than theknown pass through, or bias flow through the circuit. Flow triggeringrequires a dual inspiratory/expiratory limb circuit. At some point inthe mid to late expiratory phase, the ventilator delivers apredetermined constant flow that circulates through the inspiratory andexpiratory limb of the circuit and out through the open expiratoryvalve. With flow triggering the inspiratory valve or mechanism ispartially open in the transition phase between exhalation andinhalation, allowing low flows concurrent with the patient's inspiratoryeffort to enhance triggering sensitivity of the machine. Flow ismeasured at both the proximal connection of the inspiratory limb andnear the expiratory valve. Any drop in flow is assumed to represent thepatient's effort to breathe in gas, and the inspiratory breath istriggered. Though flow through the ventilator circuit may reduce thework the patient has to do to draw in an initial portion of the breathto trigger the ventilator, the delivered breath is still positivepressure generated and is either pressure or volume targeted.

One very different type of CSPPV mode is High Frequency Jet Ventilation(HFJV). A pulsating (non-continuous) jet is delivered via a catheterplaced within a tracheal tube with inflated cuff. The pulsing volume isdetermined by setting a driving pressure in pounds per square inch(e.g., 30 psi) and the set rate is multiples of the patient's breathingrate (e.g. 150 breaths per minute) and not synchronized with thepatient's efforts. A second source of gas flow is available from theventilator circuit that can be drawn into the tracheal tube directlythrough the patient's breathing efforts or indirectly drawn in by aventuri effect from flow through the device. Gas that passes through thecircuit and past the patient's airway must exit through, at minimum, apartially open exhalation valve. Gas exhaled by the patient must alsoexit via the exhalation valve.

HFJV is different than the present invention for a number of reasons.First, it is a form of Positive Pressure Ventilation (PPV) (i.e.,pressure-targeted). Gas is delivered in discreet boluses in a rapidmanner not synchronized with the patient respiratory cycle. It is aclosed system with the exhalation valve partially or completely openduring exhalation. Finally, a second lumen is required to deliveradditional flow to the patient.

Another technology that utilizes a catheter placed within acuff-inflated tracheal tube during concurrent CSPPV is called TrachealGas Inflation (TGI). TGI is different than the present inventionbecause, in addition to a delivered CSPPV mode via the tracheal tube, anadditional flow of gas is insufflated into the trachea via a catheter ina closed system with the cuff inflated. As with HFJV delivered withCSPPV, a second source of gas is supplied via a second lumen, and gasthat exits the patient must exit the exhalation valve. The exhalationvalve is partially or completely open during exhalation. With TGI, thesecond lumen delivers standard CSPPV breaths concurrent with flowthrough the tracheal catheter. Thus, TGI is a mode delivered inconjunction with one or more CSPPV modes.

3. Solution to the Problem

The present invention provides an open system for flow-targetedventilation to augment the respiration of a self-breathing patient. Apredetermined flow waveform is delivered to the patient's airway insynchronization with the patient's breathing cycle and at a sufficientflow rate to achieve a desired physiologic outcome, such as mitigatingpressure in the patient's airway, reducing the patient's work ofbreathing, flushing carbon dioxide from the patient's airway, andincreasing blood oxygenation. The present system can also be integratedinto CSPPV and SCSPPV devices. For example, one goal of integrating orcombining the present system with PPV in one device is to eliminate theiterative steps of switching the patient back and forth between twoseparate devices to achieve a needed clinical outcome. Another goal isto improve access of certain patient populations to the medical benefitsof the present invention while eliminating the need for capitalizationof a separate device. This controls cost, reduces redundancy of deliverydevices, increases efficiency, saves space at the patient bedside andimproves resource allocation.

The present system is intended to augment ventilation by superimposingcontinuous, non-constant and, under some conditions, non-continuousflows upon the spontaneous negative-pressure self-breathing of patients.Unlike prior art pressure-targeted or volume-targeted positive pressureventilation, this invention is flow-targeted because achievement ofspecific flows and flow waveforms are the targeted outcome.Clinician-defined flows are targeted for specific phases or componentsof phases of the patient's breathing cycle in order to achieve one ormore physiologic improvements. Unlike CSPPV or SCSPPV where positivepressure is either the targeted endpoint or an expected consequence ofvolume-targeted ventilation, the present invention uses an open systemand avoids generation of positive pressures that can cause patientdiscomfort and injury. Specially designed tubing airway devices maintainan unobstructed interface between the airway and atmosphere. A varietyof sensors can be used to detect properties associated with phases ofthe patient's breathing cycle. A microprocessor receives and processesthe data generated by the sensors for intelligent monitoring andregulation of the present system.

In the presence of respiratory distress, the invention mitigates thenegative-pressure swings that the patient with respiratory compromisemust generate during inspiration and the positive-pressure swings thatmust be generated during expiration with certain diseases and disorders.These pressure swings result from increased work of breathing (WOB). Thepresent system can mitigate the patient requirement for generatingpressure, and can thus mitigate excessive WOB, while still allowing thepatient to self-breathe in an open system without the need for CSPPV orSCSPPV. With certain diseases or disorders the patient may benefit bynot attempting to inspire through the upper airway at all, butcompensate by closing the vocal cords (glottis) and mouth and passivelyletting the device inflate the lungs at the prescribed flow and flowpattern. Though some pressure is generally encountered, it is not theprimary target of the device output and muscular work by the diaphragmand thoraco-abdominal muscles is not required to generate pressure.Thus, the system mitigates a pressure that would be generated by thepatient as a result of WOB. The free-breathing patient determines whenthe transitions between inspiration and expiration occur.

SUMMARY OF THE INVENTION

This invention provides an open system to deliver breath-synchronized,flow-targeted ventilation to augment respiration by a self-breathingpatient. A sensor detects a physical property of a patient's respiratorycycle. A processor monitors the sensor and controls a gas source todeliver oxygen-containing gas through a tube extending into thepatient's airway with the flow rate varying over each inspiratory andexpiratory phase of the respiratory cycle in a predeterminednon-constant waveform synchronized with the respiratory cycle to augmentthe patient's spontaneous respiration. Gas is delivered at a flow ratesufficient to significantly mitigate the airway pressure the patientmust generate during spontaneous breathing and thereby reduce thepatient's work of breathing.

These and other advantages, features, and objects of the presentinvention will be more readily understood in view of the followingdetailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction withthe accompanying drawings, in which:

FIG. 1 is a simplified diagram of an embodiment of the present systemusing external air and oxygen supplies.

FIG. 2 is a simplified diagram of an embodiment of the present systemthat uses an external oxygen supply but has an internalblower/compressor to supply air.

FIG. 3 is a simplified diagram of an embodiment of the present systemhaving an internal blower/compressor and an internal oxygenconcentrator.

FIG. 4 is a simplified diagram of an embodiment of the present systemhaving an internal blower/compressor and an external oxygen supply suchas a compressed gas cylinder, liquid source or oxygen concentrator.

FIG. 5 is a simplified diagram of an embodiment of the present systemhaving an internal blower/compressor and external delivery of oxygenthrough a connection into the delivery circuit leading to the patient.

FIG. 6 is a cross-sectional view of an embodiment of an airway interface60 intended for use as a tracheal catheter assembly.

FIG. 7 is a cross-sectional view of a portion of a patient's airwayfollowing insertion of the tracheal catheter assembly shown in FIG. 6.

FIG. 8 is a perspective view of an embodiment of the airway interface 60in which a tracheal catheter is assembled with a tracheostomy tube.

FIG. 9 is an exploded perspective view of the tracheostomy tube beingassembled with its inner cannula.

FIG. 10 is an exploded perspective view of the tracheal catheter beingassembled with the tracheostomy tube and inner cannula.

FIG. 11 is a vertical cross-sectional view of a patient's airwayfollowing insertion of the tracheostomy tube assembly shown in FIG. 8.

FIG. 12 is a detailed horizontal cross-sectional view of the connectorportion of the tracheostomy tube assembly in FIG. 8.

FIG. 13 is a cross-sectional view of an embodiment of the airwayinterface 60 intended for use as a nasopharyngeal catheter assembly.

FIG. 14 is a cross-sectional view of a portion of a patient's headfollowing insertion of the nasopharyngeal catheter assembly shown inFIG. 13.

FIG. 15 is a front view of a patient's head following insertion of thenasopharyngeal catheter assembly in FIGS. 13 and 14.

FIG. 16 is a cross-sectional view of an embodiment of the airwayinterface 60 intended for use as a nasal cannula assembly.

FIG. 17 is a cross-sectional view of a portion of a patient's headfollowing insertion of the nasal cannula assembly shown in FIG. 16

FIG. 18 is a front view of a patient's head following insertion of thenasal cannula assembly in FIGS. 16 and 17.

FIG. 19 is a set of graphs illustrating the respiratory mechanics with aprior art pressure-targeted CSPPV or SCSPPV.

FIG. 20 is a set of graphs illustrating the respiratory mechanics withprior art volume-targeted CSPPV.

FIG. 21 is a set of graphs illustrating the respiratory mechanics in anormal healthy person in a relaxed state.

FIG. 22 is a set of graphs showing the respiratory mechanics in apatient in respiratory distress due to an exacerbation of emphysema withbronchitis.

FIG. 23 is a set of graphs showing the respiratory mechanics for apatient with Adult Respiratory Distress Syndrome (ARDS).

FIG. 24 is a set of graphs showing the respiratory mechanics for apatient with obstructive sleep apnea with respiratory distress.

FIG. 25 is a set of graphs illustrating breathing in an emphysemapatient in respiratory distress treated using the present invention toprovide interrupted flow-targeted ventilation.

FIG. 26 is a set of graphs illustrating breathing in an emphysemapatient in respiratory distress treated using the present system with analternative waveform to deliver continuous flow-targeted ventilation.

FIG. 27 is a set of graphs illustrating breathing in an emphysemapatient in mild respiratory distress treated using the present inventionto deliver with interrupted flow-targeted ventilation.

FIG. 28 is a set of graphs depicting breathing in an ARDS patient inrespiratory distress treated with the present system to provideinterrupted flow-targeted ventilation.

FIG. 29 illustrates breathing in an obstructive sleep apnea patient inrespiratory distress treated with the present system.

FIG. 30 illustrates breathing in an emphysema patient in mildrespiratory distress treated using the present system with anotheralternative waveform to deliver uninterrupted flow-targeted ventilation.

FIG. 31 illustrates breathing in an emphysema patient in mildrespiratory distress treated with the present invention supplyinginterrupted flow-targeted ventilation with modification of thealternative waveform of FIG. 30.

FIG. 32 illustrates breathing in an emphysema patient in mildrespiratory distress treated using the present invention with continuousflow-targeted ventilation and patient control of passive inflation.

DETAILED DESCRIPTION OF THE INVENTION

A basic configuration of the present system is shown in FIG. 1. Incontrast to CSPPV devices, there is only one limb to the circuit. Thereis an inspiratory circuit, without the presence of a “Y” connector or anexpiratory circuit, or expiratory valve. Unlike SCSPPV devices, there isnot an expiratory valve located on the single limb circuit. Since theexpiratory limb is not present, the flow, pressure or other sensingdevices connected to the expiratory limb of the CSPPV device are notnecessary.

In this embodiment, the single limb inspiratory delivery circuit passesthrough a system that heats and/or humidifies the gas delivered to thepatient 10. In the preferred embodiment the gas is delivered through aservo temperature-controlled humidifier 26 with a heated breathingcircuit 27 that delivers gas within an approximate predefinedtemperature range (approximately 34-38 degrees centigrade) and relativehumidity (approximately 70 to 100%). The circuit 27 may be heated by awire, circulating water, or similar means or the tubing may be insulatedby a chamber of air or other means. Under each alternativeconfiguration, the heated and/or humidified circuit 27 may either beconnected to the patient airway interface 60 directly or through aninterposed mid-section hose 28 as illustrated in FIGS. 1-5.

The patient airway interface 60 may include a variety of tubes placedwithin the airway, including, but not limited to a tracheostomy tube,tracheal catheter, tracheal catheter within a tracheostomy tube,endotracheal tube, nasopharyngeal catheter, endotracheal catheter,endotracheal catheter within an endotracheal tube, or nasal cannula orcatheter. The patient airway interface 60 may have one or more sensorsor sampling tubing/devices that can be attached or integrated into theoutside wall of the tube. Since, unlike CSPPV where breathing onlyoccurs through the inner lumen of the cuffed tracheal tube, the presentdevice delivers flow through the lumen of the tube (e.g., a catheter),while allowing self-breathing with additional flow in and out throughthe upper airway and around the tube. Particularly during inhalation,the gas entering around the tube can be evaluated to measure or estimatethe quantity and properties of the patient's self-generated portion ofthe breath. Similarly, the gas exiting around the tube can be used tomeasure or estimate the quantity and properties of the patient's exhaledbreath, particularly with physiologic measurements such as CO₂concentration. Devices such as an oximeter 31 or tissue CO₂ sensors canbe attached to the airway interface 60, in particular where the devicecomes in contact with skin or airway mucosal surface or similar bodysurfaces (such as the tissue interface of a tracheal stoma). This isdiscussed in greater detail below with regard to FIGS. 6-18. As analternative embodiment, one or more sensors or sampling tubing/devicescan be attached or integrated into the inner wall of the tube orconnection of either the mid-section hose 28 or heated circuit 27 to thetube. Measurements could include internal pressure at the distal end ofthe device and the delivered temperature, humidity, flow or F₁O₂ orother gas properties (e.g., nitric oxide or helium concentration). Oneor more sensors or measuring devices can be located on, within oradjacent to the patient airway interface 60 as described above, and datacan be electronically transferred back to the device (wired or wirelesstransmission of various forms). Examples include an ultrasonic probe,light emitting device, oximeter 31 or tissue CO₂ probe, thermistor 32 orother flow sensing device. Alternatively, a sampling tubing 30 can beused to transfer pressure back into the ventilator system 20 formeasurement using a pressure transducer 33, as illustrated for examplein FIGS. 1-5. When the purging pump 37 is disengaged by a valve or othermechanism, a valve assembly or similar device can additionally bypassthe aspiration pump 36 and pressure within the gas sampling tube 30 canbe in free communication with the pressure transducer 33. The pressuredifferential can be used to determine flow using an additionaltransducer 34. The sampling tubing 30 can be used to draw sampled gasback into the device for measurement through use of an aspiration pump36 or some similar mechanism with optional periodic purging of the linewith air or a liquid (e.g., saline or water) using a positive pressurepump 37 or similar purging device. The aspiration pump 36 would deliverthe sample to sensor/measurement devices within the present device.Examples of sensor/measuring devices include a helium, nitric oxide,oxygen or CO₂ analyzer 35.

Data generated by sensors at or near the patient airway interface 60 anddata generated by sensors within the ventilator system 20 from samplescollected at or near the airway interface 60 and data generated fromsensors, such as an oxygen analyzer 29, pressure transducer 24 and flowtransducer 23 incorporated into the gas delivery mechanism of the system20 are electronically transferred to the processor 21 throughanalog-to-digital conversion as needed, so digital information eitherreaches the processor or is converted from analog to digital at theprocessor 21. The processor 21 is typically in bidirectional or two-waycommunications with the entire sensing/measurement system (includingsensors, aspiration and purging systems). The processor 21 also governsany necessary valve control, output regulation, calibration, qualitycontrol or operation status and self-test or auto-regulationinformation. In particular, at least one of the sensors measures aphysical property (e.g., pressure, flow or carbon dioxide level)associated the patient's respiratory cycle. It should be understood thatthe processor 21 can be a microprocessor, controller or any othersuitable type of hardware with sufficient intelligence to monitor thesensors, detect a desired phase (or phase component) of the patient'srespiratory cycle, and control the ventilation system to deliver apredetermined flow profile of oxygen-containing gas varying over eachinspiratory and expiratory phase of the respiratory cycle.

As another embodiment of the invention (not shown), one or more of thesensor/measurement devices, aspiration and purging systems and relatedhardware/software can be external and removably attached to the presentdevice with appropriate ports 38 and 39 to connect the device or devicesto communicate with the processor 21 and sensors measurement devices andsampling tubing located on or adjacent to the patient airway interface60. Furthermore, devices in communication with the present device couldinclude monitors such as pulse oximeters and tissue CO₂ monitors. Inaddition to sensor and measurement devices at or adjacent to the patientairway interface 60, other sensor and measurement devices can beintegrated within the delivery system of the present device.

Oxygen-containing gas can be made available to the delivery system froma variety of gas sources, as shown in FIGS. 1-5. For example, FIG. 1illustrates an external oxygen supply 51 from potentially a number ofsources (including, but not limited to piped wall oxygen, direct liquidor compressed gas source, or concentrator). There is also an externalsource 52 of air (such as piped wall air, direct air compressor orblower source) or other medical gases including, but not limited tohelium or nitric oxide by a variety of delivery means). Though notlimited to this application, the system would likely be used in ahospital or similar institutional setting.

FIG. 2 demonstrates an external oxygen source 51 similar to FIG. 1, Incontrast, an air compressor, blower or similar air source 53 is housedwithin the present system. Though not limited to this application, thesystem would also likely be used in a hospital or similar institutionalsetting.

FIG. 3 shows delivery of air by an internal air compressor or blower 53or similar air generating device as noted in FIG. 2. However, oxygen issupplied by an internal oxygen concentrator 54 or comparable oxygengenerating device that is housed within the present system. Thisembodiment could be use in either the home or a hospital or similarinstitutional setting.

FIG. 4 illustrates the embodiment of an internal air compressor orblower 53 or similar air generating device and external delivery ofoxygen into the proximal limb of the internal flow delivery systemthrough use of an external compressed gas cylinder 55, liquid source,concentrator or comparable system. This embodiment may be mostappropriate for a setting such as the home or nursing home setting.

FIG. 5 denotes an internal air compressor or blower 53 or similar airgenerating device and external delivery of oxygen with a T-connection oran equivalent connection into the delivery circuit distal to the presentsystem. Sources could be (but not limited to) an external compressedoxygen gas cylinder or liquid oxygen source or oxygen concentrator. Thisembodiment may also be most appropriate for a setting such as the homeor nursing home setting. Whether generated by examples illustrated inFIGS. 1-5 or variations thereof, the gas composition from thoserespective sources is regulated by one or more gas supply valves 56.Examples are shown in FIGS. 1-5 that regulate the mixing ofconcentrations and proportionate flow into the system. Concentrations ofgas delivery can be confirmed by analyzers for the appropriate sourcesuch as oxygen, helium or nitric oxide.

The inspiratory valve 22 noted in FIGS. 1-5 regulates the maximum orpeak flow and the associated flow waveform under the control of theprocessor 21. Flow transducers 23 can be placed proximal, distal orpreferably both proximal and distal to the inspiratory valve 22. Apressure transducer 24 or other pressure sensing/measuring device ispreferably located proximal to the inspiratory valve 22 to measurepressure and detect excessive machine pressure. Additionally, thepreferred embodiment also incorporates a pressure transducer 24 or otherpressure sensing/measuring device that is preferably located distal tothe inspiratory valve 22 to measure pressure and detect excessivepressure within the flow delivery circuit. As shown in FIGS. 1-5, aconduit before and a conduit after the inspiratory valve merge into aconduit that has a pressure relief valve 25 that vents to theatmosphere, preventing excessive pressure build up within the presentdevice or within the flow delivery circuit. Other configurationsaccomplishing the same outcome apply. Data from all of the sensors,valves, monitoring, measurement devices or other systems are passed onto the processor 21 and the processor 21 is preferably in bidirectionalcommunication with those devices and multiple communications can occursimultaneously among the processor 21 and other systems.

The processor 21 is also in two-way or bidirectional communication witha local and optional remote graphic user interface 40 (GUI) or similardevice with control panel, and with a local or optional remote audioalarm system 42. The GUI display 40 allows the user to set flow-targetedparameters, including peak flow, or any of the instantaneous flowwaveform characteristics that can be targeted for a respiratory phase orcomponent of a respiratory phase. Respiratory phases include aninspiratory phase, a transition phase from inspiratory to expiratory, anexpiratory phase, and a transition phase from expiratory to inspiratory.Additionally, there are components to both the inspiratory andexpiratory phases. In one embodiment, those flow-targeted peak flows(e.g., inspiratory and expiratory or optional peak transition flows) andrelative flow waveform examples of described flows targeted to phases orcomponents of the phases can be graphically presented to the user asoptions (among other flow pattern options) for selection. Selected“Help” screens could walk the user through various decision trees, suchas selection of phase-related peak flows and flow patterns based uponspecific management goals. The operator GUI 40 or other controlinterface allows the user to assess, measure, monitor, adjust or alterany parameter chosen by the user. The primary targeted parameter is peakflow and the associated flow pattern. However, the user can adjust forsecondary parameters including, but not limited to delivered gas oxygenconcentration, as well as the concentration of other medical gases suchas air, helium, or nitric oxide, and the delivered temperature andhumidity.

Though the system is flow targeted, every ventilation system must havesecondary, or fail safe back-ups. As with other ventilation systems,excessive internal pressures within the present device or within theflow delivery circuit can be measured. Similarly, sensors, measuringdevices and/or gas sampling tubing attached to, or associated with theairway interface device 60 can integrate with the present system tosense over-pressurization within the patient airway. The GUI interface40 can allow the user to select default or custom pressure limits, andpressure exceeding that limit at any point will be dissipated (e.g.,through the pressure relief valve noted in FIGS. 1-5) with theappropriate audible alarm 42 and visual alarm. The intelligent processor21 can utilize data from all valves, sensors, measurement devices orother systems integrated within or in communication with the presentdevice to perform calibration, quality assurance checks, other automatictests or evaluations and to make automatic adjustments and compilereports.

The processor 21 can use data from the sensors, measurement devices orsampling tubing on, within or adjacent to the patient airway interfacedevices 60 to determine the phases and components of phases of therespiratory cycle of the self-breathing patient 10. Examples include,but are not limited to, flow (e.g., thermistor or differential pressureassessment), airway pressure and airway CO₂ waveform analysis. AirwayCO₂ waveform analysis is a preferred embodiment, especially for trachealtubes and tracheal catheters, as sampling of gas near the carina caneliminate a substantial portion of physiologic or “wasted” dead space, aknown confounding factor in end-tidal capnography accuracy. The waveformwill not only identify phases and components of phases of therespiratory cycle, but can serve as real time breath-by-breath analysisand/or trending of the adequacy of ventilation in the self-breathingpatient 10 supported by the present invention.

Assessment of flow (and related volume) during inspiration that iscontributed by the present invention and also self-breathed by thepatient 10 would be clinically useful. The flow delivered by the presentsystem during the time of inspiratory phase (T_(I)) and related volumecan be calculated by the processor 21. Qualitative flow drawn in throughthe upper airway can be assessed by a thermistor attached to the outsideof the airway tube. Alternatively, two pressure-sensing tubes ofdifferent lengths along the outside of the tube allow the system tomeasure differential pressure. This can be used by the processor 21 topresent a qualitative assessment of flow drawn in through the upperairway. Given the patient characteristics (e.g. sex, height, race, ageor other parameters), the processor 21 can calculate the area of thetrachea based on known anatomical relationships. With additional inputof the tracheal tube external diameter, the processor 21 can calculatethe area between the outside of the tube and the tracheal wall throughwhich the patient 10 is breathing between the two pressure measurements.The processor 21 can model and calculate the flow (and related volume)during inspiration that is contributed by the self-breathing patient 10.As an alternative, one or more ultrasonic probes can be placed on theoutside of the tube to measure and calculate the area between thetracheal mucosa and tube. In summary, these measurements, along with theknown properties of the tube (such as the surface area of anyfenestrations) could be used with the pressure differential to calculateflow, and integrate flow into volume. As needed, corrections can be madefor temperature, humidity, gas concentration and other factors. Usingthe same principle, flow and volume of the exhaled gas can be assessed.Various processor calculations of physiologic parameters can bepresented to the user through the GUI 40 to present therespiratory-cardio physiologic status of the patient 10, including suchassessments as volumetric carbon dioxide. Acceptable ranges can be setby the user, with GUI 40 and audio alarms 42 set to alert exceptions.With the intelligent processor 21, device monitoring information andphysiologic data can input into a feedback loop that allows theinvention to make specific adjustments based upon monitoring andphysiologic data criteria determined by the user. The user can setappropriate limits with an appropriate local or remote GUI 40, andaudible alerts and alarms 42.

FIGS. 6 and 7 show an embodiment in which the airway interface tube 60is a tracheal catheter assembly. FIG. 6 is a cross-sectional view of thetracheal catheter assembly. FIG. 7 is a cross-sectional view of aportion of a patient's airway 10 following insertion of the trachealcatheter assembly. The diameter of the tracheal catheter 72 should beselected to provide an adequate flow of oxygen-containing gas to achievephysiologic benefits such as significantly mitigating the airwaypressure the patient must generate during spontaneous breathing andthereby reduce the patient's work of breathing. However, the diameter ofthe tracheal catheter 72 should be sufficiently small so as not tosubstantially interfere with the patient's spontaneous respirationaround the tracheal catheter. The distal portion of the trachealcatheter 72 can be equipped with a variety of sensors (e.g., athermistor 67) for sensing selected physical properties associated withthe patient's respiratory cycle. Additionally, a sensor such as anoximeter sensor 66 placed adjacent to tissue in the neck, as illustratedin FIG. 7, can measure the patient's blood oxygen level similar to pulseoximetry. Sensors can be placed in any desired position or orientationwith respect to the catheter 72 and are connected by sensor lines andconnector 70, 71. In addition, one or more sampling tubes 30 extendingalong the tracheal catheter 72 enables sensors in the ventilation system20 to monitor physical properties associated with the patient'srespiration in the airway. The tracheal catheter 72 is held in placerelative to the patient's trachea by means of a flange 65 and securingnecklace 69 extending around the patient's neck.

FIGS. 8-12 show an embodiment of the airway interface 60 in which atracheal catheter 72 is inserted through a tracheostomy tube 80. FIG. 8is a perspective view of the assembly of the tracheostomy tube 80 as amodification of a conventional SHILEY™ tracheostomy tube marketed byTyco Healthcare Group LP, Nellcor Puritan Bennet Division, ofPleasanton, Calif. The tracheostomy tube 80 has an insertable innercannula 84 with a 15 mm connector 86 on its proximal end to allowattachment to a standard ventilator. The inner cannula 84 also has twosecuring grips 85 that engage ridges 83 on the proximal end of thetracheostomy tube 80 to removably secure the inner cannula 84 to thetracheostomy tube 80. FIG. 9 is an exploded perspective view of theinner cannula 84 being inserted through the tracheostomy tube 80. Itshould be noted that the tracheostomy tube 80 with its inner cannula 84can continue to be used in the conventional manner, if desired, toventilate the patient with a CSPPV ventilator. In this mode, theinflatable cuff 81 at the distal end of the tracheostomy tube can beinflated by means of a pilot balloon 82 to occlude the patients'trachea.

However, the embodiment of the airway interface 60 shown in FIGS. 8-12also enables the present invention to be retrofit to a tracheostomy tubeto provide a second mode of ventilation as an alternative to CSPPV. Toillustrate this second mode, which is the subject of this invention,FIG. 10 is an exploded perspective view the tracheal catheter 72 beinginserted through the tracheostomy tube 80 and its inner cannula 84. FIG.11 is a vertical cross-sectional view of a patient's airway followinginsertion of the tracheostomy tube assembly shown in FIG. 8, with thecuff 81 deflated so as not to substantially interfere with the patient'sspontaneous respiration around the tracheostomy tube 80.

The cap 87 on the proximal end of the tracheal catheter 72 fits over theconnector 86 on the end of the inner cannula 84 of the tracheostomy tube80. The cap 87 also includes an 11 mm connector 89 for attachment to themid-section hose 28 (illustrated in FIGS. 1-5) leading from theventilator system 20 in the present system. The outside diameter of thetracheal catheter 72 is typically much less than the inside diameter ofthe inner cannula 84 of the tracheostomy tube 80. This enables thepressure in the patient's airway to be monitored by the ventilatorsystem 20 via a sampling tube connected to a pressure monitoring port 88in the cap 87. FIG. 12 is a detail horizontal cross-sectional view ofthe connector portion of the tracheostomy tube assembly in FIG. 8.

The tracheostomy tube in FIGS. 8-11 is modified for the present systemwith a gas sampling tubing 67 extending along a portion of the tube.Additionally, an oximetry sensor 66 is attached to a portion of thetracheostomy tube. A variety of the other types and combinations ofsensors and additional gas sampling tubings may be used. Alternatively,the tracheostomy tube and inner cannula as illustrated in FIG. 9 may beemployed as the airway interface 60 for the present system withoutcombined use with the tracheal catheter. The cuff is deflated and the 15mm connector 86 on the inner cannula 84 is attached to a 15 mm connectoron a mid-section hose 28 or directly to the heated circuit inspiratorylimb 27 illustrated in FIGS. 1-5 equipped with a 15 mm interfaceconnector. The present system can also be used with fenestrations (notshown) placed in the tracheostomy tube and inner cannula illustrated inFIGS. 8-11. Other tracheostomy tube designs modified with one or moresensors can also be used with the present system. Additionally, othertracheostomy tube designs modified with one or more gas sampling tubingscan be used with the present system.

FIGS. 13-15 show an embodiment of an airway interface 60 intended foruse as a nasal tube, such as a nasopharyngeal catheter assembly. FIG. 13is a cross-sectional view of a nasopharyngeal catheter assembly. In thisembodiment, the airway interface 60 includes a nasopharyngeal catheter62 designed for insertion through a patient's nostril into thenasopharynx as shown for example in the cross-sectional view provided inFIG. 14. In this embodiment, the tip of the catheter extends into theoropharynx, but could terminate in the nasopharynx. FIG. 15 is a frontview of a patient's head following insertion of the nasopharyngealcatheter assembly. The catheter 62 includes a radio-opaque stripe 65 toguide insertion with x-ray imaging. Following insertion, a flow ofoxygen-containing gas is supplied through the nasopharyngeal catheter 62via connecting tubing 63 extending beneath the patient's nose at a flowrate sufficient to significantly mitigate the airway pressure thepatient must generate during spontaneous breathing and thereby reducethe patient's work of breathing. The diameter of the catheter 62 is oflesser concern since it does not interfere with spontaneous breathingthrough the mouth. A thermistor 67 on a distal portion of the cathetercan be used to monitor air flow in the pharyngeal airway. Electricalleads 71 run from the thermistor 67 along the catheter 62 and connectingtubing 63 to the ventilator system 20 to enable the processor 21 tosynchronize the delivered flow of oxygen-containing gas to the patient'srespiratory cycle. The embodiment of the airway interface 60 shown inFIGS. 13-15 also includes a pressure monitoring/gas sampling tube 30 asa second lumen parallel to the nasopharyngeal catheter 62. Optionally,an oximeter 66 on a distal portion of the catheter 62 contacts themucosal tissue on the nasal cavity, as shown in FIG. 14, to monitor thepatient's blood oxygen saturation. It should be noted that other typesof nasal tubes could be substituted in place of a nasopharyngealcatheter.

FIGS. 16-18 show another embodiment in which the airway interface 60 isa nasal cannula assembly. This embodiment is similar to that shown inFIGS. 13-15, but employs two shorter nasal cannulae 62 as the nasaltubes, in place of a single, long nasopharyngeal catheter. Here again, anumber of sensors (e.g., an oximeter 66 or thermistor 67) and pressuremonitoring/gas sampling tubes can be placed on the nasal tubes 62 tomonitor the patient's respiration.

Examples of Use. FIG. 19 illustrates pressure, flow and volume waveformswith breathing cycles experienced by a patient receiving prior artpressure-targeted CSPPV or SCSPPV. Time is on the horizontal axis. Thereare four phases to the respiratory cycle. There is a transition phasebetween expiration and inspiration, which is followed by the inspiratoryphase. Similarly, there is a transition phase between inspiration andexpiration which is followed by the expiratory phase. The inspiratoryand expiratory phases also have different components. Prior artpressure-targeted CSPPV and SCSPPV devices and methods are designed totake over the patient's normal spontaneous negative pressureself-breathing. For example, in FIG. 19 this patient is receiving acommonly prescribed targeted pressure of 5 cm H₂O during end expiration,or positive end-expiratory pressure (PEEP). During the beginning of theinspiratory phase, the patient makes an effort to spontaneously negativepressure breathe, which results in a transient drop in the appliedpositive pressure to approximately 3 cm H₂O, but not to a normalnegative value. A series of pressure-targeted breaths are triggered eachtime the patient attempts to normally breathe, and the positive pressureventilator will override the patient's natural efforts and will force,or pressurize the breath to exactly achieve the targeted maximalinspiratory pressure of 25 cm H₂O. Once the targeted pressure isreached, the exhalation valve opens and allows pressure to drop onexhalation, but the valve then closes when the targeted expiratorypressure of 5 cm H₂O is reached. On a breath-by-breath basis, themaximum inspiratory flow and flow delivery patterns vary. A breath witha longer inspiratory time alters flow delivery and achieved tidalvolume, even though targeted pressure is unchanged. Similarly, themaximum expiratory flows and flow patterns vary even though a targetedPEEP is achieved and maintained. Peak inspiratory flows and flowpatterns are relatively independent of the target inspiratory pressure.Peak expiratory flows and flow patterns are relatively independent ofthe target expiratory pressure.

FIG. 20 illustrates implementation of a prior art volume-targeted CSPPV.The ventilator has a targeted tidal volume of 800 ml which is achievedwith each breath. With a closed system and the mechanical properties ofthis patient's lungs (resistance, compliance, etc.) delivery of thetargeted volume results in generation of 30 cm H₂O at peak inspiration,and the pressure dissipates only when the exhalation valve opensallowing the patient to exhale. In addition to the targeted inspiredvolume, there is a commonly used expiratory targeted pressure, which is5 cm H₂O of PEEP that is maintained by closure of the expiratory valve.The patient is not making any efforts to self-breathe, and negativepressure deflections below the PEEP level are not seen. Consequently,because of the closed system and absence of self-breathing, the targetedvolume is achieved with each breath and no variations in pressure, flowor flow patterns are seen in this steady state. Had self-breathingefforts occurred with volume-targeted CSPPV, variability in peakpressure and both peak flow and flow patterns would have been observed.

FIG. 21 illustrates respiratory mechanics in a normal negative-pressureself-breathing healthy person in a relaxed state. This is representativeof how individuals spontaneously breathe when independent from either apositive pressure or negative pressure mechanical ventilator. In short,individuals self-generate a negative or sub-atmospheric pressure thatdraws the breath into the lungs. During the inspiratory phase, theperson uses respiratory muscles to generate negative pressure. Since thelungs are healthy, minimal work of breathing (WOB) is required to drawadequate flow into the lungs. At about mid-inspiration the amount ofnegative pressure as well as flow into the lungs has reached the peak,and values begin to return to the baseline of zero pressure and flow(sinusoidal pattern). During the transition phase between inspirationand expiration there is a slight pause where negative pressure hasdissipated, inspiratory flow has ceased and no additional volume hasentered the lungs. During the exhalation phase, the elastic recoil ofthe lungs and chest wall is enough to cause the gas flow to carry theinspired volume out of the lungs and into the atmosphere undernegligible resistance. Minimal positive pressure is generated and littleor no expiratory WOB is done. Again, expiratory pressure and flow occurin a sinusoidal pattern. There is also a brief period of zero pressureand zero flow in the transition phase between expiration and inspirationwhere no volume exchange occurs. The ratio of inspiration to expirationis approximately 1:1.5, which is an efficient pattern that maintains anormal respiratory rate and adequate time for exhalation with normallungs.

The following discussions and accompanying FIGS. 22-24 present examplesof pathophysiology of a number of diseases and disorders that maybenefit from use of the present invention. Application of the inventionis by no means limited to these examples of diseases and disorders.

FIG. 22 shows contrasting respiratory mechanics in a self-breathingpatient in respiratory distress due to an exacerbation of emphysema withbronchitis. Increased airway resistance resulting from bronchial airwayobstruction directly increases inspiratory WOB. The over-distendeddiseased lung is difficult to inflate and inspiratory WOB is increased.Consequently, the airway pressure curve swings significantly morenegative throughout the sinusoidal inspiratory phase, due to increasedinspiratory WOB. Patients have difficulty drawing the breath down intothe deep alveolar regions of the lungs where oxygen uptake occurs. Sincethe respiratory muscles in emphysema patients do not perform normally,there is a limit to how much extra work can be performed. Thoughpressure may transiently return to zero during the phase betweeninspiration and expiration, the airways are so collapsed and obstructedthat significant expiratory WOB is required to allow the trapped breathto be exhaled. Patients purse their lips and close their vocal cords(which do not require much energy) and then forcefully engage theirexpiratory respiratory muscles to build up back-pressure required tomechanically dilate the airways so that obstruction can be improved andexhalation can more effectively occur in this disease state.Consequently, expiratory pressures are elevated even during normalnegative pressure breathing that occurs without positive pressuremechanical ventilation. The patient also tries to allow more time fortrapped gas to be exhaled, so even though the time required forinspiration is little changed, proportionately more time is spent inexhalation (1:2 ratio). This requires a slower respiratory rate. If thiscan not occur, air trapping (hyperinflation) results. This inefficientbreathing pattern causes worsening gas exchange and mechanics andfurther increases in WOB.

In addition to requirements for increased inspiratory and expiratoryWOB, other physiologic derangements in patients with emphysema arehypoxemia, increased physiologic dead space and reduced alveolarventilation. Destruction of the alveoli (air sacs) and related bloodvasculature and airway disease impair the effectiveness and efficiencyof gas exchange, resulting in reduced uptake of oxygen and eliminationof carbon dioxide. Due to the disease, patients have mismatch where theareas of ventilation don't adequately match blood flow, so inadequateoxygen enters the body (hypoxemia). Additionally, there are manybronchial tubes that lead to diseased alveolar sacs where there isventilation, but completely inadequate blood flow. Consequently,ventilation is wasted and there is increased dead space due tocompletely inadequate gas exchange. Consequently, for a given tidalbreath in, a higher than normal portion of it does not get to thealveolar sacs where oxygen can be taken up and carbon dioxide can bereleased from the blood stream (inadequate alveolar ventilation).Additionally, during the last component of the expiratory phase, some ofthe carbon dioxide does not get exhaled into the atmosphere and istrapped in the airways (trachea, bronchial tubes, pharynx, oral andnasal cavity) and alveolar sacs without blood flow (physiologic deadspace). Patients with increased physiologic dead space, as in thisexample, have more trapped carbon dioxide that is breathed in to thealveolar sacs again during the first component of the next inspiratoryphase. The self-breathing patient has few choices; either increase therespiratory rate and/or tidal volume in an effort to try to get moreminute ventilation to functioning alveolar sacs (this requires an evenfurther increase in WOB), or to give in to excessive WOB and retaincarbon dioxide in the blood (develop worsening respiratory acidosis, orrespiratory failure). The present system is uniquely positioned toimprove or correct these physiologic abnormalities while still allowingthe patient to spontaneously self-breathe without CSPPV or SCSPPV. Thispresentation of a patient with respiratory distress due to anexacerbation of emphysema is intended to illustrate one end of thespectrum of respiratory compromise with one example of a disorder wherespecific physiologic abnormalities occur and can be tied to a specificphase or component of a phase in the self-breathing cycle.

Negative-pressure self-breathing in a neurologic or neuromusculardisease patient with respiratory distress should also be considered.Patients with spine or brain injury and those with neuromusculardisorders can have significant respiratory distress due to impairedneurologic respiratory drive to breathe or due to the fact that therespiratory muscles are unable to generate adequate WOB. The respiratorymechanics would have a similar pattern to the healthy person in FIG. 21except that adequate negative pressures may not be sustained duringnegative pressure self-breathing. Consequently, air flow and the tidalvolume decrease. The low tidal volume results in a high dead space totidal volume ratio, and functioning alveolar sacs receive inadequatealveolar ventilation. Elevated carbon dioxide and low blood oxygenlevels can result. Mismatches in blood flow and gas in alveolar sacs canfurther compromise blood oxygen levels. The present invention isuniquely positioned to improve or correct these physiologicabnormalities while minimizing required WOB and still allowing thepatient to spontaneously self-breathe without CSPPV or SCSPPV.

FIG. 23 illustrates a patient on the other end of the spectrum ofrespiratory compromise with one example of a disorder called AdultRespiratory Distress Syndrome (ARDS). The self-breathing pattern isdifferent than FIG. 21. The ARDS patient has some common features withthe patient in FIG. 22, but also some very different pathophysiologicderangements. Unlike the over-stretched and poorly elastic lung inemphysema, ARDS causes a very stiff lung that is difficult to inflateand the lungs have blood flow that is shunted around alveolar sacs thatare collapsed or full of fluid (congestive atelectasis). Consequently,very little oxygen gets to the lungs and the patient is driven tobreathe deep and fast to attempt to get more oxygen into functionalalveolar sacs to compensate. Though still inspiring with a normalsinusoidal negative pressure swing during the inspiratory phase ofbreathing, the negative pressure pattern is pronounced due to the highinspiratory WOB required to inflate the stiff lungs with the highventilatory requirements. Any reduction in either anatomic orphysiologic dead space would be beneficial in reducing excessiveventilatory requirements. Patients have difficulty drawing the breathdown into the deep functioning alveolar regions of the lungs whereoxygen uptake can occur. Intense WOB is required. Though the elasticrecoil of the stiff lung helps gas initially escape during the earlyexpiratory phase, expiratory WOB (particularly during the later segmentsof the expiratory phase) is increased to force the gas out of the lungsso the expiratory time can be shorter (1:1 ratio) allowing a fasterrespiratory rate without significant compromise of the relationship ofinspiration to the total breathing cycle (respiratory duty cycle).Pressure also swings positive during the expiratory phase as patientshave increased expiratory WOB in an effort to force flow duringexpiration into collapsed alveolar sacs (atelectasis) for lungrecruitment. The inspiratory and expiratory WOB are further driven bythe respiratory center's intense stimulus to drive higher tidal volumesand faster respiratory rates. The present invention is uniquelypositioned to improve or correct these physiologic abnormalities whilestill allowing the patient to spontaneously self-breathe without CSPPVor SCSPPV.

FIG. 24 shows respiratory mechanics during negative-pressureself-breathing in a patient with obstructive sleep apnea withrespiratory distress. A normal respiratory cycle during sleep whereobstruction is not present is illustrated on the left. It is similar toFIG. 21. However, in an iterative cyclic fashion, the upper airwaytotally obstructs, resulting in the absence of inspiratory flow andabsence of inspiratory volume. Large negative pressure values aregenerated as the patient struggles to inspire. Similarly, the patientforcefully attempts to exhale against the obstructed upper airway andsignificant expiratory pressures are generated, but flow is curtailedand there is no inspired tidal volume to exhale. The obstruction isaggravated because upper airway tissue is “sucked” together by thestronger and stronger negative pressure efforts and “obstruction begetsobstruction” because nothing is stenting the opposing tissues to keepthem apart as negative pressure efforts increase. Abnormalities inoxygen and carbon dioxide exchange occur and cardiovascular andneurologic impairment with severely disrupted sleep architecture areproblematic.

Continuous Positive Airway Pressure (CPAP), which is a form of SCSPPV,uses pressure to prevent obstruction with sleep apnea patients and toprevent large negative pressure swings. Similarly, the present system isuniquely positioned to improve or correct these physiologicabnormalities while still allowing the patient to spontaneouslyself-breathe without the need for CPAP and associated discomforts andcomplications encountered with SCSPPV. Sleep apnea patients can havecentral episodes, where there are iterative periods throughout sleepwhere no efforts are made to breathe. Patients have breathing cycleswith no upper airway obstruction, but the absence of flow, volume andpressure are noted. The problems are getting adequate oxygen deep intothe alveolar units where oxygen uptake can occur and getting carbondioxide expelled into the atmosphere. The present invention is uniquelypositioned to improve or correct these physiologic abnormalities whilestill allowing the patient to spontaneously self-breathe without theneed for CPAP and associated discomforts and complications encounteredwith SCSPPV.

The following discussions and FIGS. 25-32 serve to specificallydemonstrate how using an open system to provide pressure-mitigating,breath-synchronized, flow-targeted ventilation can improve physiology inself-breathing patients with the previously described diseases anddisorders. As stated previously, use of the invention is not limited tothese disease and disorder examples. Furthermore, these example figuresare not intended to limit the scope of the invention. It should be notedthat the flow waveforms and associated flow rates of oxygen-containinggas delivered through the airway interface 60 should be sufficient toachieve the desired physiological benefit for the patient, such asreducing the patient's work of breathing by reducing the airway pressurethat the patient must generate during spontaneous breathing, flushingcarbon dioxide from the patient's airway, and increasing ventilation andimproving blood oxygenation. This typically requires a peak flow rate inthe approximate range of 7 to 60 L/min for adults, and proportionallyreduced peak flow rates for pediatric and infant populations. Theinspiratory and expiratory flow waveforms and related flow ratesassociated with phases of the inspiratory and expiratory respiratorycycle are examples only. Required flow rates and waveforms may changefrom time to time in the management of an individual. Similarly,required flow rates and waveforms will vary based upon the management ofadult, child or infant populations.

FIG. 25 illustrates negative-pressure self-breathing in an emphysemapatient in respiratory distress treated using the present invention toprovide interrupted flow-targeted ventilation (Example 1). FIG. 25 andothers that follow show two respiratory cycles of the previous examplesof impaired respiratory mechanics in patients with respiratory distressdue to different respiratory disorders with specific pathophysiologicderangements that have been previously defined. A key element is thatthe present invention supports the normal self-breathing process whileeither eliminating or minimizing problems encountered with prior artsystems.

As previously mentioned, there are four phases to the respiratory cycle.There is a transition phase between expiration and inspiration, which isfollowed by the inspiratory phase. Similarly, there is a transitionphase between inspiration and expiration which is followed by theexpiratory phase. Furthermore, there are components within theinspiratory and expiratory phases. The flow, pressure and volumegenerated with the patient's unsupported self-breathing in FIG. 25 areillustrated on the vertical axis in solid lines. Intervention with peakflows and flow patterns delivered by the invention that are superimposedupon the respiratory cycle of the self-breathing patient in an opensystem is demonstrated in dashed lines. Expected clinical response withrespect to alterations of patient pressure patterns achieved as a resultof superimposed targeted flows delivered by the invention is shown indashed lines. Dashed lines also reflect the anticipated increases intidal volume resulting from use of the invention. Other anticipatedphysiologic outcomes of the invention are discussed. In this particularexample there is interrupted flow delivery in the transition betweenexpiration and inspiration and between inspiration and expiration thatmatches a normal breathing pattern.

FIG. 25 shows a rapidly accelerating inspiratory flow with a peak of 40cm H₂O. The initial accelerated flow is synchronized with the patient'sinitial inspiratory effort in the very first component of theinspiratory phase. The early onset of a high flow that exceeds therequirement of the normal breathing pattern facilitates delivery of gasdeep into functional alveolar gas exchange units which results inimproved alveolar ventilation and consequently improved oxygen uptakeand carbon dioxide elimination. Flow during the very early component ofthe inspiratory phase has maximum impact upon oxygen delivery duringself-breathing. Following the accelerated inspiratory flow during theearly inspiratory phase, the pattern transforms into a convexdecelerating pattern that overlays a sinusoidal flow pattern of thepatient's breath during mid to late inspiration. The rapidlyaccelerating peak inspiratory flow (+40 L/min peak in FIG. 25) and flowpattern reduces inspiratory WOB because the device delivers flow on theleading edge of the breath and less respiratory muscular work isrequired to physically draw the gas into the lungs. The deceleratingflow pattern superimposed upon the patient's diminishing flow supportsthe diminishing needs for work to be performed during the remainder ofthe inspiratory phase. The inspiratory flow supplied by the presentsystem also enhances alveolar ventilation during this phase of thepatient's respiratory cycle and tidal volume is increased.

FIG. 25 demonstrates a reduction in the inspiratory negative-pressureswing, which indicates reduced inspiratory WOB. In other words, thenegative pressure required by the patient to inspire is mitigated by useof the device's targeted flow pattern. Because of the open design of thesystem, positive pressure during inspiration does not occur because anygas that is not inhaled can easily escape into the atmosphere,mitigating positive pressure buildup. At the onset of expiration, a flowof 15 L/min is triggered in this example and a rectangular flow patterncontinues through early and mid exhalation. Patients with emphysemapurse their lips and vocal cords throughout exhalation (which requiresnegligible work) and then use the work of the expiratory muscles tobuild up back-pressure to mechanically dilate diseased airways tofacilitate exhalation. The expiratory flow and flow pattern delivered bythe device mechanically dilates the diseased airways and mitigates thepressure that the patient would otherwise generate by increased WOB.During the late component of exhalation the peak flow with therectangular flow pattern is increased to 25 L/min. This flow boostcontinues to further mechanically dilate the airways to prevent distalairway collapse, but also flushes out the carbon dioxide that collectsin the anatomic and physiologic dead space areas at the end ofexhalation. Carbon dioxide is washed out and replaced by oxygen enrichedgas that will be available to functioning alveoli on the next breath.

Additional flow provided by the invention during exhalation allows theself-breathing patient more effective and efficient use of theexpiratory muscles, vocal cords, pharynx and lips to facilitate normalquality, non-fatiguing speech. Similarly, additional flow provided bythe invention during exhalation allows the self-breathing patient toincrease cough effectiveness by increasing flow during the expulsivephase of cough. Additional flow provided by the invention duringexhalation allows the self-breathing patient more effective andefficient use of the vocal cords and lips in maximizing the physiologiceffects related to the rate at which gas exits the chest. Different flowrates and flow patterns administered during the expiratory phase thatare illustrated in the following examples may also result in thesebenefits in a variety of patient populations.

FIG. 26 illustrates negative-pressure self-breathing in an emphysemapatient in respiratory distress treated using the present system todeliver continuous flow-targeted ventilation (Example 2). The onlydifference in application of the invention between the patientmanagement in FIG. 25 versus FIG. 26 is that the flow is not interruptedduring the transition between exhalation and inhalation or in thetransition between inhalation and exhalation. Based upon apatient-specific condition for a variety of disorders, continuous flowmay or may not be advantageous. Continuous flow may also be used withany of the examples demonstrating interrupted flow.

FIG. 27 illustrates negative-pressure self-breathing in an emphysemapatient in mild respiratory distress treated using the present inventionto deliver interrupted flow-targeted ventilation (Example 3). In thisexample of the implementation of the invention the patient is determinedby the physician to be less compromised and requires less aggressivesupport. The application delivers a peak flow and flow pattern to mimicthe sinusoidal inspiratory and expiratory flow patterns of theself-breathing patient. Reduced WOB on inspiration and expiration occur,alveolar ventilation is supported, and airway collapse is treated.Similarly, this flow-targeted ventilation with this flow pattern islikely to be beneficial for self-breathing patients with neurologic orneuromuscular diseases. The physiologic derangements in this patientpopulation have been previously described. These individuals shouldbenefit from the present invention.

FIG. 28 depicts negative-pressure self-breathing in an ARDS patient inrespiratory distress treated with the present system to provideinterrupted flow-targeted ventilation (Example 4). FIG. 28 shows arapidly accelerating inspiratory flow (with a peak of 45 cm H₂O in thisexample). The initial accelerated flow is synchronized with thepatient's initial inspiratory effort in the very first component of theinspiratory phase. The early onset of a high flow that exceedsrequirements of the normal breathing pattern facilitates delivery of gasdeep into functional alveolar gas exchange units, which results inimproved ventilation to alveolar sacs causing improved oxygen uptake andcarbon dioxide elimination. Flow during the very early component of theinspiratory phase has maximum impact upon oxygen delivery duringself-breathing. Failure of adequate uptake of oxygen in spite ofadministration of a gas with a high percentage of oxygen (refractoryhypoxemia) is a derangement in ARDS that should be improved by thepresent invention, particularly with this flow pattern that is alsodesigned to recruit collapsed alveolar sacs. Following the acceleratedinspiratory flow during the early inspiratory phase, the patterntransforms into a convex decelerating pattern that overlays sinusoidalflow pattern of the patient's breath during mid to late inspiration. Therapidly accelerating peak inspiratory flow and flow pattern reducesinspiratory WOB because the device delivers flow on the leading edge ofthe breath and less respiratory muscular work is required to physicallydraw the gas deep into the lungs. The first portion of the inspiratoryphase in ARDS requires the most WOB because the stiff lungscharacteristic of this disorder are most stiff (highest elastic recoil)at lowest lung volumes encountered at the beginning of inspiration. FIG.28 demonstrates a reduction in the inspiratory negative pressure swing,which indicates reduced inspiratory WOB. In other words, the negativepressure required by the patient to inspire is mitigated by use of thedevice's targeted flow pattern. Because of the open design of thesystem, positive pressure during inspiration does not occur because anygas that is not inhaled can easily escape into the atmosphere,mitigating positive pressure buildup.

Patients with ARDS, due to the high elastic recoil created by thedisorder, are generally able to passively exhale gas from the lungs.However, with the tendency of alveolar sacs to collapse, administrationof flow during exhalation can be beneficial in preventing furtheratelectasis (alveolar collapse) or even opening collapsed alveolar sacs(recruitment). The ARDS patient requires a high minute ventilation.Though excessive physiologic dead space may not be present, anyreduction in physiologic and/or anatomic dead space can reduceventilatory requirements during self-breathing. The elevated flowachieved at end-expiration with this flow-targeted pattern is designedto meet those needs through carbon dioxide wash out.

FIG. 29 (Example 5) illustrates negative-pressure self-breathing in anobstructive sleep apnea patient in respiratory distress treated with thepresent system. Upper airway collapse and the physiologic derangementsin obstructive sleep apnea have been previously described. In thetreatment of obstructive sleep apnea, the present system can be used totarget a flow pattern with flow rates that maintain patency or opennessof the upper airway during self-breathing. The flow rate (and pattern)required to achieve and maintain patency or openness may be differentrelative to the phase or component of the phase of the respiratory cycleand requirements may vary from individual to individual.

FIG. 29 illustrates an example where an initial high flow at the onsetof inspiration occurs to prevent upper airway collapse during theinitial negative pressure generated at the onset of inspiration. Thoughthe flow pattern tapers during mid to late inspiration, relatively highflows are maintained to prevent inspiratory upper airway collapse, whichresults in increase inspiratory WOB. The mitigation of increasedinspiratory negative pressures prevents obstruction from begettingobstruction. Similarly, relatively high flows are maintained during theexpiratory phase though flows are of less magnitude. These flows alsostent the airway during exhalation and prevent the floppy upper airwaytissues from causing obstruction. Accelerated flow occurs towards theend of exhalation in order to maintain patency prior to the onset of thenext negative pressure swing at the onset of inspiration. This is anexample where continuous flow, rather than interrupted flow, may be thepreferred method as it may be more effective in preventing upper airwaycollapse in obstructive sleep apnea. Unlike Bi-level Positive PressureVentilation or Continuous Positive Airway Pressure (SCSPPV systems) apartial obstruction between the upper airway and atmosphere is notrequired. Patients self-breathe with an open system. Complications anddiscomforts of SCSPPV systems are avoided. Relief and prevention ofobstruction prevents the physiologic derangements associated with thedisorder. For patients with central sleep apnea, flow patterns shown inFIGS. 25 and 26 should be effective in improving the physiologicderangements.

FIG. 30 (Example 6) illustrates negative-pressure self-breathing in anemphysema patient in mild respiratory distress treated using the presentsystem to deliver uninterrupted flow-targeted ventilation. In thisexample of an implementation of the invention, the patient is alsodetermined by the physician to be less compromised and requires lessaggressive support. However, the support is designed to augmentself-breathing. A flow of 15 L/min is selected to be administeredthroughout the inspiratory phase and a flow of 7 L/min is selected to beadministered throughout the expiratory phase. Thus, flow-targetedventilation is synchronized with the respiratory cycle and results in aflow pattern that is not the same constant flow throughout the entirerespiratory cycle. The higher inspiratory flow is designed to augmentthe inspiratory breath and the lower expiratory flow is designed tofacilitate speech and glottic functioning and to prevent airway collapseand wash out dead space without providing excessive expiratory flows forthis particular patient. Flow is uninterrupted during transitionsbetween inspiration and expiration and between expiration andinspiration.

FIG. 31 illustrates negative-pressure self-breathing in an emphysemapatient in mild respiratory distress treated with the present inventionsupplying interrupted flow-targeted ventilation (Example 7). Similar tothe example in FIG. 30 with the implementation of the invention, thepatient is also determined by the physician to be less compromised andrequires less aggressive support. However, the support is designed toaugment self-breathing. A flow of 15 L/min is selected to beadministered throughout the inspiratory phase and a flow of 7 L/min isselected to be administered throughout the expiratory phase. Thus,flow-targeted ventilation is synchronized with the respiratory cycle andresults in a flow pattern that is not constant throughout the entirerespiratory cycle. The higher inspiratory flow is designed to augmentthe inspiratory breath and the lower expiratory flow is designed tofacilitate speech and glottic functioning, to prevent airway collapseand wash out dead space without providing excessive expiratory flows forthis particular patient. The difference is that flow is interruptedduring transitions between inspiration and expiration and betweenexpiration and inspiration.

FIG. 32 illustrates negative-pressure self-breathing in an emphysemapatient in mild respiratory distress treated using the present inventionwith continuous flow-targeted ventilation and patient control of passiveinflation (Example 8). As noted previously, patients with lung diseasemay use their vocal cords to control or regulate flow in and out of thelungs. Additionally, patients may also “purse” or close their lips tocontrol respiratory flow. This requires little effort. Certain patientsmay benefit if they learn to close their vocal cords and purse theirlips on inspiration, and rather than using negative pressure generatedthrough WOB by the respiratory muscles, they would allow the flow of gasfrom the present system to passively and effortlessly inflate the lungs.Unlike CSPPV where the device determines when the breath is triggered onor inspiration is cycled off, the self-breathing patient controls therespiratory cycle.

FIG. 32 demonstrates an emphysema patient in respiratory distress where,due to closure of the vocal cords or mouth on inspiration, the flow fromthe device passively inflates the lungs. Negative pressure otherwiserequired to inflate the lungs by the self-breathing patient'srespiratory muscles is mitigated. The flow pattern is similar to FIG. 26where the fast ramp-up allows the patient to promptly inflate the lungs,allowing more time for exhalation. Adequate time to exhale isbeneficial. Little or no work is required by the diaphragm or otherinspiratory muscles. Though positive pressure is achieved oninspiration, no pressure delivered by the device is targeted and thepatient determines when the pressure is relieved by opening the vocalcords and lips. Partial closure of the lips and vocal cords during theexpiratory phase and resulting physiologic benefits have been described.Other targeted inspiratory flows and flow patterns may be beneficial inthis patient population.

The above disclosure sets forth a number of embodiments of the presentinvention described in detail with respect to the accompanying drawings.Those skilled in this art will appreciate that various changes,modifications, other structural arrangements, and other embodimentscould be practiced under the teachings of the present invention withoutdeparting from the scope of this invention as set forth in the followingclaims.

We claim:
 1. An apparatus adapted for delivering a flow ofoxygen-containing gas to the airway of a patient with spontaneousrespiration, said apparatus comprising: a tube adapted for delivering aflow of oxygen-containing gas into a patient's airway withoutinterfering with a patient's spontaneous respiration; a gas sourcehaving a housing and delivering a variable flow of gas through the tubecombining oxygen from an oxygen source and air from an air source; asensor adapted for detecting a physical property of a patient'srespiratory cycle; and a processor monitoring the sensor and controllingthe gas source to deliver a flow of oxygen-containing gas through thetube adapted to augment the patient's spontaneous respiration, said flowvarying over each inspiratory and expiratory phase of the respiratorycycle in a predetermined non-constant flow waveform synchronized withthe respiratory cycle, said waveform including: (a) a positive flowaccelerating at the onset of the patient's inspiratory phase at a flowrate sufficient to significantly mitigate the airway pressure thepatient must generate during spontaneous breathing and thereby reducethe patient's work of breathing; and (b) a positive flow during at leastthe early portion of the patient's expiratory phase at a flow ratesufficient to significantly mitigate the airway pressure the patientmust generate during spontaneous breathing and thereby reduce thepatient's work of breathing, and to wash carbon dioxide from thepatient's airway.
 2. The apparatus of claim 1 wherein the oxygen sourcecomprises an oxygen concentrator within the housing of the gas source.3. The apparatus of claim 1 wherein the oxygen source is external to thehousing of the gas source.
 4. The apparatus of claim 1 wherein the airsource comprises an air compressor within the housing to the gas source.5. The apparatus of claim 1 wherein the air source is external to thehousing of the gas source.
 6. The apparatus of claim 1 wherein thesensor is attached to a portion of the tube adapted for placement in apatient's airway.
 7. The apparatus of claim 1 further comprising asampling tube extending along a portion of the tube adapted forplacement in a patient's airway to the sensor.
 8. The apparatus of claim1 wherein the sensor comprises a pressure transducer.
 9. The apparatusof claim 1 wherein the sensor comprises a flow sensor.
 10. The apparatusof claim 1 wherein the sensor comprises a thermistor.
 11. The apparatusof claim 1 wherein the sensor comprises a carbon dioxide sensor.
 12. Theapparatus of claim 1 wherein the tube comprises a nasopharyngealcatheter.
 13. The apparatus of claim 1 wherein the tube comprisescannula.
 14. The apparatus of claim 1 wherein the processor interruptsthe flow for a portion of the respiratory cycle.